Generally, there exist various fluid jet methods by which a fluid, such as ink, is ejected onto a target (printing medium). Here, the following description explains an ink jet printing method in which the ink is used as the fluid.
As drop on demand ink jet printing methods, (i) a piezo printing method in which a piezoelectric phenomenon is utilized, (ii) a thermal printing method in which a film boiling phenomenon of ink is utilized, and (iii) an electrostatic attraction printing method in which an electrostatic phenomenon is utilized, etc are developed. Especially, in recent years, a high-resolution ink jet printing method is strongly demanded. In order to realize the high-resolution ink jet recording, it is indispensable to reduce the size of the ink droplet to be ejected.
Here, the movement of the ink droplet, which is ejected from the nozzle and lands on the printing medium, is expressed by a motion equation (Equation (1)).ρink·(4/3·πd3)·dv/dt=−Cd·(½·ρair·v2)·(π·d2/4)  (1)
The above ρink is a volume density of ink, V is a volume of a droplet, v is a velocity of a droplet, Cd is a drag coefficient, ρair is an air density, and d is a radius of an ink droplet. Cd is expressed by Equation (2).Cd=24/Re·(1+ 3/16·Re0.62)  (2)
Re is a Reynolds number. Re is expressed by Equation (3).Re=2·d·ρink·v/η,  (3)where η is an air viscosity.
The influence exercised by the radius of the droplet on the movement energy of the ink droplet of the left side of Equation (1) is greater than the influence exercised by the radius of the droplet on the viscous resistance of the air. On this account, when the velocity of the droplet is constant, the smaller the droplet becomes, the more quickly the velocity of the droplet decreases. As a result, the droplet may not be able to reach the printing medium separated in a predetermined distance. Even when the droplet reaches the printing medium, the positioning accuracy of the droplet is low.
In order to prevent these from occurring, it is necessary to increase an initial velocity of the ejected droplet, that is, it is necessary to increase an ejection energy per unit volume.
However, according to the conventional piezo ink jet head and the conventional thermal ink jet head, the following problems occur when the size of the ejected droplet is decreased, that is, when the ejection energy of the droplet per unit volume is increased. It was especially difficult to set the amount of the ejected droplet to be equal to or less than 1 pl, that is, difficult to set the diameter of the droplet to be equal to or less than Φ10 μm.
Problem (A): The ejection energy of the piezo ink jet head relates to the amount of displacement and a developed pressure of a piezoid to be driven. The amount of displacement of the piezoid inseparably relates to the amount of the ink ejected, that is, to the size of the ink droplet. In order to reduce the size of the droplet, it is necessary to reduce the amount of displacement. It is difficult to improve the ejection energy, per unit volume, of the ejected droplet.
Problem (B): The thermal ink jet head utilizes the film boiling phenomenon of ink. Pressure generated when bubbles are formed is physically limited. Moreover, the ejection energy is substantially determined by the area of a heating element. The area of the heating element is substantially in proportion to a volume of the bubble formed, that is, in proportion to the amount of ink ejected. On this account, by decreasing the size of the ink droplet, the volume of the bubble formed is decreased and the ejection energy is also decreased. Therefore, it is difficult to improve the ejection energy, per unit volume, of the ejected droplet of the ink.
Problem (C): In both the piezo printing method and the thermal printing method, how much the drive element (heating element) works relates closely to the amount of ink ejected. Therefore, in the case of ejecting extremely minute droplet, it is very difficult to suppress the variation of the size of the droplet.
Here, as a method for solving the above problems, a method of ejecting minute droplets by using the electrostatic attraction printing method has been developed.
In the electrostatic attraction printing method, a motion equation of the ink droplet ejected from the nozzle is expressed below as Equation (4).ρink·(4/3·π·d3)·dv/dt=q·E−Cd·(½·ρair·v2)·(π·d2/4),  (4)where q is the amount of electric charge of a droplet, and E is a peripheral electric field intensity.
According to Equation (4), in the electrostatic attraction printing method, the ejected droplet receives, in addition to the ejection energy, an electrostatic force while the droplet is flying. Therefore, it is possible to reduce the ejection energy per unit volume and possible to apply the method to the ejection of a minute droplet.
As an ink jet device using such an electrostatic attraction printing method (hereinafter referred to as “electrostatic attraction ink jet device”), Document 1 (Japanese Laid-Open Patent Publication No. 238774/1996 (Tokukaihei 8-238774, published on Sep. 17, 1996)) discloses an ink jet device in which an electrode for applying voltages is provided inside the nozzle. Moreover, Document 2 (Japanese Laid-Open Patent Publication No. 127410/2000 (Tokukai 2000-127410, published on May 9, 2000)) discloses an ink jet device which has a slit as a nozzle, is provided with a stylus electrode protruded from the nozzle, and ejects ink containing fine particles.
The following description explains the ink jet device disclosed in Document 1 in reference to FIG. 17. FIG. 17 is a schematic cross section of the ink jet device.
In FIG. 17, 101 is an ink ejection chamber, 102 is ink, 103 is an ink chamber, 104 is a nozzle hole, 105 is an ink tank, 106 is an ink supplying path, 107 is a rotating roller, 108 is a printing medium, 110 is a control element portion, and 111 is a process control section.
Further, 114 is an electrostatic field applying electrode portion which is provided on the ink chamber 103 side in the ink jet chamber 101, 115 is a counter electrode portion which is a metallic drum provided at the rotating roller 107, and 116 is a bias power supply portion for applying a negative voltage of thousands of volts to the counter electrode portion 115. 117 is a high voltage power supply portion for supplying a high voltage of hundreds of volts to the electrostatic field applying electrode portion 114, and 118 is a ground portion.
Here, between the electrostatic field applying electrode portion 114 and the counter electrode portion 115, the negative voltage of thousands of volts applied from the bias power supply portion 116 to the counter electrode portion 115 and a high voltage of hundreds of volts from the high voltage power supply portion 117 are superimposed. In this way, a superimposed electric field is generated. The ejection of the ink 102 ejected from the nozzle 104 is controlled by means of the superimposed electric field.
In addition, 119 is a projected meniscus which is formed at the nozzle hole 104 by the bias voltage of thousands of volts applied to the counter electrode portion 115.
The following description explains an operation of the electrostatic attraction ink jet device thus arranged.
First, the ink 102 passes through the ink supplying path 106 by the capillary phenomenon, and is transferred to the nozzle hole 104 which ejects the ink 102. At this time, the counter electrode portion 115, to which the printing medium 108 is mounted, is provided face to face with the nozzle hole 104.
The ink 102 reached the nozzle hole 104 forms the projected ink meniscus 119 by the bias voltage of thousands of volts applied to the counter electrode portion 115. A signal voltage of hundreds of volts is applied from the high voltage power supply portion 117 to the electrostatic field applying electrode portion 114 which is provided in the ink chamber 103. The signal voltage thus applied is superimposed on the voltage applied from the bias power supply portion 116 to the counter electrode portion 115. Then, by the superimposed electric field, the ink 102 is ejected onto the printing medium 108. As a result, a printed image is formed.
The following description explains movement of the meniscus, until the droplet is ejected, of the droplet of the ink jet device disclosed in Document 1 in reference to FIGS. 18(a) to 18(c).
As illustrated in FIG. 18(a), before a drive voltage is applied, a projected meniscus 119a is formed on the surface of the ink because of the balance between (i) the electrostatic force of the bias voltage applied to the ink and (ii) the surface tension energy of the ink.
As illustrated in FIG. 18(b), when the drive voltage is applied, the electric charge generated on the fluid surface starts to concentrate on the center of the fluid surface. As a result, a meniscus 119b is so formed that the center of the fluid surface is highly projected.
As illustrated in FIG. 18(c), when the drive voltage is continuously applied, the electric charge generated on the fluid surface further concentrates on the center of the fluid surface. This results in the formation of a meniscus 119c which is a semilunar shape called “taylor cone”. When the electrostatic force of the electric charge concentrated on the top of the taylor cone exceeds the surface tension energy of the ink, a droplet is formed and ejected.
Next, the following description explains the ink jet device disclosed in Document 2 in reference to FIG. 19. FIG. 19 is a diagram illustrating a schematic arrangement of the ink jet device.
As illustrated in FIG. 19, a case of the present ink jet device contains (i), as an ink jet head, a line-shaped recording head 211 formed by using low dielectric materials (acrylic resin, ceramics, etc.), (ii) a counter electrode 210 which is made of metal or high dielectric materials and is provided face to face with an ink-ejecting opening of the recording head 211, (iii) an ink tank 212 for storing ink which is made by dispersing electrified pigment particles in nonconductive ink medium, (iv) ink circulating system (pumps 214a and 214b, pipings 215a and 215b) for circulating ink between the ink tank 212 and the recording head 211, (v) a pulse voltage generating device 213 which applies a pulse voltage, for ejecting an ink droplet which forms one pixel of a record image, to each ejection electrode 211a, (vi) a drive circuit (not illustrated) which controls the pulse voltage generating device 213 according to an image data, (vii) a printing medium feeding apparatus (not illustrated) which causes a printing medium A to pass through a space between the recording head 211 and the counter electrode 210, (viii) a controller (not illustrated) which controls the entire device, etc.
The ink circulating system is composed of (i) two pipings 215a and 215b each of which connects the recording head 211 with the ink tank 212 and (ii) two pumps 214a and 214b which are driven by the controller.
The ink circulating system is divided into (i) an ink supplying system which supplies ink to the recording head 211 and (ii) an ink collecting system which collects ink from the recording head 211.
In the ink supplying system, the ink is pumped up by the pump 214a from the ink tank 212, and the ink thus pumped up is delivered to the ink supplying portion of the recording head 211 through the piping 215a. Meanwhile, in the ink collecting system, the ink is pumped up by the pump 215b from the ink collecting portion of the recording head 211, and the ink thus pumped up is compulsorily collected to the ink tank 212 through the piping 215b. 
Moreover, as illustrated in FIG. 20, the recording head 211 includes (i) an ink supplying portion 220a which spreads the ink, supplied from the piping 215a of the ink supplying system, so that the ink is spread to be as wide as a line, (ii) an ink flow path 221 which guides the ink, supplied from the ink supplying part 220a, so that the ink forms a mountain-shape, (iii) an ink collecting portion 220b which connects the ink flow path 221 with the piping 215b of the ink collecting system, (iv) a slit-shaped ink-ejecting opening 222 which is open to the counter electrode 210 at the mountaintop of the ink flow path 221 and has an appropriate width (approximately 0.2 mm), (v) a plurality of ejection electrodes 211a provided in the ink ejection opening 222 with a predetermined pitch (approximately 0.2 mm), and (vi) party walls 223 which are made of low dielectric materials (for example, ceramic) and are provided on both sides and an upper surface of each ejection electrode 211a. 
Each of the ejection electrodes 211a is made of metals, such as copper, nickel, etc. On the surface of the ejection electrode 211a, a low dielectric film (for example, polyimide film), which excels in wettability, for preventing pigments from being adhered is formed. Moreover, the top of each ejection electrode 211a is formed like a triangular pyramid. Each ejection electrode 211a projects from the ink-ejecting opening 222 to the counter electrode 210 by an appropriate length (70 μm to 80 μm).
According to the controller, the above-described drive circuit (not illustrated) gives a control signal to the pulse voltage generating device 213 during a time corresponding to gradation data included in the image data. Then, the pulse voltage generating device 213 superimposes a pulse Vp, whose pulse top corresponds to the kind of the control signal, on the high voltage signal which is on the bias voltage Vb so as to output a pulse voltage thus superimposed.
When the image data is transferred, the controller drives two pumps 214a and 214b of the ink circulating system. Then, the ink is delivered from the ink supplying portion 220a, and the negative pressure is applied to the ink collecting portion 220b. The ink flowing in the ink flow path 211 passes through the gap between the party walls 223 by the capillary phenomena. Then, the ink spreads so as to reach the top of each ejection electrode 211a. At this time, the negative pressure is applied to the surface of each ink fluid near the top of the ejection electrode 211a. Therefore, the ink meniscus is formed on the top of each ejection electrode 211a. 
Further, the controller controls the printing medium feed mechanism so that the printing medium A is fed in a predetermined direction. Moreover, by controlling the drive circuit, the high voltage signal is applied between the printing medium A and the ejection electrode 211a. 
The following description explains the movement of the meniscus, until the droplet is ejected, of the droplet of the ink jet device disclosed in Document 2 in reference to FIGS. 21 to 24.
As illustrated in FIG. 21, when the pulse voltage generated by the pulse voltage generating device 213 is applied to the ejection electrode 211a in the recording head 211, an electric field, which goes from the ejection electrode 211a to the counter electrode 210, is generated. Here, because the ejection electrode 211a whose top is sharp is used, the strongest electric field is generated around the top of the ejection electrode 211a. 
As illustrated in FIG. 22, when such an electric field is generated, each electrified pigment particle 201a in the ink solvent moves toward the surface of the ink fluid by the force fE (FIG. 23) exerted from the electric field. In this way, the density of pigment around the surface of the ink fluid is increased.
As illustrated in FIG. 23, when the density of pigment is thus increased, a plurality of electrified pigment particles 201a around the surface of the ink fluid starts to cohere at the opposite side of the electrode. Then, a pigment aggregate 201 starts to grow to form a spherical shape near the surface of the ink fluid. Then, the electrostatic repulsive force fcon from the pigment aggregate 201 starts to influence each electrified pigment particle 201a. That is, each electrified pigment particle 201a is influenced by the total force ftotal which is a resultant force of the electrostatic repulsive force fcon from the pigment aggregate 201 and the force fE from the electric field E generated by the pulse voltage.
Therefore, in the case in which the electrostatic repulsive force between the electrified pigment particles does not excess the force of cohesion of the electrified pigment particles, when the force fE exceeds the electrostatic repulsive force fcon (fE≧fcon), the electrified pigment particles 201a form the pigment aggregate 201. Note that, the force fE is applied from the electric field to the electrified pigment particle 201a (electrified pigment particle 201a which is located on a straight line between the top of the ejection electrode 211a and the center of the pigment aggregate 201) to which the total force ftotal in a direction of the pigment aggregate 201 is applied.
The pigment aggregate 201 formed by n pieces of electrified pigment particles 201a receives an electrostatic repulsive force FE from the electric field E generated by the pulse voltage, and also receives the binding force Fesc from the ink solvent. When the electrostatic repulsive force FE and the binding force Fesc are balanced, the pigment aggregate 201 becomes stable in a state in which the pigment aggregate 201 projects slightly from the surface of the ink fluid.
Further, as illustrated in FIGS. 24(a) to 24(c), when the pigment aggregate 201 grows and the electrostatic repulsive force FE exceeds the binding force Fesc, the pigment aggregate 201 is separated from the surface 200a of the ink fluid.
Incidentally, according to the principle of the conventional electrostatic attraction printing method, the meniscus is projected by concentrating the electric charge on the center of the meniscus. The curvature radius of a taylor cone tip portion thus projected is determined by the amount of concentrated electric charge. When the electrostatic force of the amount of concentrated electric charge and the electric field intensity exceeds the surface tension energy of the meniscus, the droplet starts to be ejected.
The maximum amount of electric charge of the meniscus is determined by the physical-property value of the ink and the curvature radius of the meniscus. Therefore, the minimum size of the droplet is determined by the physical-property value of the ink (especially, the surface tension energy) and the intensity of the electric field generated at the meniscus portion.
Generally, the surface tension energy tends to become lower in a fluid containing solvents than in a pure solution. Because typical ink contains various solvents, it is difficult to increase the surface tension energy. On this account, the ink surface tension energy is considered to be constant, and a method of decreasing the size of the droplet by increasing the electric field intensity is used.
Therefore, according to the principle of the ejection of the ink jet device disclosed in each of Documents 1 and 2, a field whose intensity is high is generated at the meniscus region whose area is much larger than a project area of the ejected droplet. By the field, the electric charge is concentrated on the center of the meniscus. Then, by an electrostatic force of the concentrated electric charge and the electric field, the ejection is carried out. Therefore, it is necessary to apply an extremely high voltage of about 2000 V. On this account, it is difficult to control the driving, and there is a problem in view of the safety of the operation of the ink jet device.
Especially, when the electric field whose intensity is high is generated in a large region, it is necessary to set the electric field intensity to be equal to or less than the intensity of the discharge breakdown (for example, the intensity of the discharge breakdown of the air between the parallel flat plates is 3×106 V/m). Therefore, the possible size of the minute droplet is fundamentally limited.
In addition, because the electric charge moves to the center of the meniscus portion, the amount of time for the electric charge to move influences the response of ejection. This causes a problem in the improvement of the print speed.
As is used in Documents 1 and 2, a method of solving these problems is (i) a method of reducing a drive voltage by applying a bias voltage which is lower than an ejection voltage, or (ii) an arrangement in which, as disclosed in Document 2, an electrode projects from a nozzle portion so that the concentration of electric charge is accelerated. Moreover, for example, as is disclosed in Document 1, a method of applying a positive voltage to ink in order to project a meniscus in ahead is also proposed.
However, both methods disclosed in Documents 1 and 2 cannot fundamentally solve the problems. Especially, when the bias voltage is applied, only one of positive and negative drive voltages can be applied. When the printing medium is made of an insulating material, the surface electric potential of the printing medium is increased by the adhesion of the electrified ejected droplet. Therefore, the positioning accuracy deteriorates. On this account, it is necessary to take countermeasures, such as eliminating, while printing, the surface potential of the printing medium.
Moreover, because the field whose intensity is high is generated at the meniscus region whose area is large, it is necessary to accurately position the counter electrode. In addition, because the dielectric constant and the thickness of the printing medium influence the positioning of the counter electrode, the degree of freedom is low when using printing mediums. Especially, when the printing medium is thick, the counter electrode has to be placed at a position remote from the electrode of the nozzle portion. On this account, it is necessary to apply a higher voltage. Moreover, many of printing mediums are difficult to be used practically.
Therefore, according to the conventional electrostatic attraction ink jet device (electrostatic attraction fluid jet device), there is a problem in that it is impossible to realize a recording device which has high resolution, is safe and is highly versatile.
The present invention was made to solve the above problems, and an object of the present invention is to provide an electrostatic attraction fluid jet device which can realize the recording device which has high resolution, is safe and is highly versatile.